This is part of a basic science introduction to mitochondria. In later posts I will be addressing the pathophysiology and treatment of neurodegenerative diseases (among other things) which will have a lot to do with the functioning of mitochondria.
Mitochondria: An Introduction part 1
(Artwork by Odra Noel www.odranoel.eu)
Mitochondria as many may remember from high school biology are the organelles (essentially “organs” of the cell) that act as the power plants or powerhouses of the cell. This is because they help in the conversion of dietary energy substrates (the macronutrients we ingest) into high energy molecules (such as ATP) whose breakdown provides the energy to run the biochemical reactions necessary for life. These energy substrates include glucose, amino acids, fatty acids, and ketone bodies. I will address the process of mitochondrial metabolism/cellular respiration in a future post.
Mitochondria are unique in that they are essentially aerobic (oxygen consuming) bacteria that were absorbed and incorporated into eukaryotic cells between 1 and 2 billions years ago. This is called the endosymbiotic theory. Chloroplasts in plant cells are similar, they are believed to be photosynthetic cyanobacteria that were incorporated into eukaryotic cells.
This gives them some interesting abilities compared to other organelles. Mitochondria can replicate on their own, independent of the cell’s replication. They have their own genome and machinery to make proteins for themselves, independent of the nucleus. These genes are mainly involved in producing the enzymes responsible for energy metabolism. However most of the original mitochondrial genome has been incorporated into the nuclear DNA. The majority of mitochondrial proteins are actually made in the cytosol and then transported to the mitochondria through specific targeting sequences on the protein (kind of like a cellular shipping address).
Mitochondria are inherited maternally, which means that all the mitochondria present in your cells are actually derived from the mitochondria of your mother. This is because the mitochondria present in the fertilized egg that were contributed by the sperm are destroyed and only those from the mother are left to populate the cells of the developing embryo.
The lack of meaningful genetic recombination and continuous maternal lineage allows us to trace our species’ origin (our most common recent ancestor) to the heart of Africa some 200,000 years ago.
An overriding principle in biology is that function follows structure. A hammer and saw may both be made of metal and rubber but they have completely different functions because of their unique structures. So let’s consider the structure of mitochondria.
Mitochondria are membrane bound organelles with a unique structure. They have two membranes with differing phospholipid and protein compositions. The outer membrane (OM) is similar to the cell’s own plasma membrane in terms of the protein and phospholipid composition. It contains porins which form aqueous channels that allow small molecules to enter. It also contains transporter proteins, called TOMMs (Translocase of the Outer Mitochondrial Membrane), which can transport larger molecules and proteins into the mitochondria. The inner membrane (IM) is where all the fun happens. It contains a much higher percentage of proteins than the OM because it is involved in the production of ATP. It also contains the unique phospholipid cardiolipin which contains 4 fatty acids rather than the normal 2, this may help in making the IM more impermeable to ions. The inner membrane folds in on itself creating finger like projections called cristae, these act to increase the surface area of the IM similar to microvilli of epithelial cells.
The Intermembrane space is the area between the OM and IM, ions and other molecules can be concentrated here. The matrix is the area inside the IM, it contains metabolic enzymes, RNA, mtDNA, and ribosomes.
Most people may be familiar with the depiction of mitochondria as sausage or bean shaped but they actually take on many different forms including long connected tubular like structures.
This is accomplished by fission and fusion events and this will become important later on.
Most cells have mitochondria, only a few types do not, such as red blood cells. Depending on the energy needs of the cell it can have 1 or it could have hundreds or thousands of mitochondria. Very energy demanding cells such as neurons necessitate many thousand mitochondria.
They are not static isolated structures as imagined from textbook figures, they are dynamic and ever evolving to suit the energy demands of the cell. They are often associated with cytoskeletal elements such as microtubules which allow them to move around the cell like a train on a track. This is especially important in the long axons and dendrites of neurons.
Another more recent discovery about the structure of mitochondria is the close association that they can have with the endoplasmic reticulum (ER), called the mitochondria-associated ER membrane or MAM.
The main function and the one that most people are familiar with is ATP production but they actually have many other important functions. Mitochondria are also involved in the storage and buffering of Ca²+ (calcium ions). Calcium is actually a very important signalling molecule in the cell. Low concentrations are maintained in the cytosol by the cell by actively pumping it out of the cell or into the ER and mitochondria. This allows any increase to be a meaningful signal. At high concentrations it can be toxic to the cell which is another reason that it is important that mitochondria are able to buffer excess calcium. Mitochondria can also participate in cell signaling through the reactive oxygen species (ROS) that are created during metabolism.
Mitochondria are involved in regulating the cell cycle and play a key role in the apoptosis pathway, a type of programmed cell death.
And just for the sake of completeness: some parts of the steroid hormone, heme synthesis and urea cycle pathways take place in mitochondria.
Well, there you have it folks. Now you know the basics of what mitochondria are and what makes them so unique. Hopefully your understanding is now more nuanced than mitochondria are just a powerhouse of the cell. In later posts I will be exploring why mitochondria are so important in human health and disease.